In general, in an iron foundry and a steel foundry, a desulfurization treatment is performed between a pig iron making process and a steel making process so as to remove sulfur content (impurity element) causing embrittlement from molten iron. In the desulfurization treatment, a desulfurizing agent such as calcined lime, calcined soda, carbide, caustic soda, and hydrated lime is supplied to a container such as a ladle in which the molten iron is received and is mixed by an impeller (mixing blade) immersed in the molten iron in the container so as to promote the desulfurization reaction therein.
As a structure for performing the desulfurization treatment, for example, a structure disclosed in the JP 2003-82409 A below and the like may be exemplified. That is, in the structure of the related art, an overload reserve amount as a ratio between a heating amount of a current operation and a heating amount when the motor is operated at 100% is estimated from the load current value of the motor rotationally driving the impeller. Then, when the estimated overload reserve amount is smaller than a predetermined threshold value, the rotational speed of the impeller is decreased, so that the impeller is rotated at the maximum rotational speed in which the operation is not stopped due to the overload of the motor. This control is used to prevent the occurrence of defective desulfurization as much as possible by setting the rotational speed of the impeller 3 to a large value.
Further, determination of the rotational speed of the impeller based on an abrasion state empirically estimated from the usage count of the impeller is also carried out although this is a company technique not disclosed as a Patent Document.